Rat cDNA libraries - large inserts required

Help, please! I have tried various libraries from various sources in an
expression cloning experiment in Xenopus oocytes. None of these libraries
(average insert size 1.0-1.5 kb) have given me any joy and I am wondering
if what I am looking for is a large protein and therefore not represented
in the libraries. Before I start out on the long road of mRNA fractionation
and library construction, is there anybody with either
a) a suitable rat (or possibly human) liver cDNA library from which large
proteins have been cloned (or are known to be represented in the library).
or
b) knowledge about where I might look for one.
If anyone else is having problems with expression cloning using Xenopus
oocytes I'd be interested in sharing thoughts, problems, etc.
Thanks in advance.
Dave West